The “Realstruktur” of the System La_2-xSr_xCu_1-yRu_yO_4-δ Studied by Rietveld and Extended X-ray Absorption Fine Structure Spectroscopy

Abstract

The structure of the layered perovskite-related system La2 - x Sr x Cu1 - y Ru y O4 - δ was investigated by Rietveld refinement calculations of X-ray diffraction data and EXAFS analysis of the Ru−K and Cu−K absorption edge. Both methods complement one another as they yield information about long-range ordering and the local environment of a specific element, respectively. Rietveld calculations have revealed remarkable changes of the interatomic distances with the composition, e.g., the degree of elongation of the Cu/Ru−O6 octahedra. To get more detailed information about the local structure we have performed EXAFS studies for selected samples with y = 0, 0.2, 0.4, 0.6, 0.8, and 1. We were able to achieve consistent results both from the Ru−K and Cu−K edge data. As an element specific method, extended X-ray absorption fine structure (EXAFS) spectroscopy is capable of distinguishing between different possible arrangements of the La/Sr and Cu/Ru atoms. We found that La and Sr are distributed statistically on the A cation position. For the B cations a small enrichment of Cu in the Ru surrounding (and vice versa) could be detected. For a given value of y this enrichment increases with increasing La content. This indicates a partial ordering of Ru and Cu within the perovskite layers. Oxygen vacancies (δ > 0) were found to be located exclusively in the Cu surrounding. Coordination numbers less than 5 reveal that a part of the copper ions is coordinated in a square planar way. Distances derived from EXAFS and XRD data show a good agreement; still, the former are usually smaller by 0.01−0.03 Å. The distance Cu−La/Sr was found to be smaller by 0.05 Å than the distance Ru−La/Sr. Apart from this, no significant differences could be detected for the distances between the various combinations of elements.

The distribution of the doped holes in La2−xSrxCu1−yRuyO4−δ

“…Sr 2 RuO 4 is the first non-copper-oxide superconductor with the same K 2 NiF 4 [4][5][6], which can be considered to be hole-doped via substitution of La by Sr and electron-doped by the substitution of Ru for Cu. Two previous studies [7,8] indicate that single phase La 2Àx Sr x Cu 1Ày -Ru y O 4Àd exists in the regions 0 6 x 6 2 and 0 6 y 6 1 with x P 2y and x < 1 þ 2y, which are illustrated in the schematic phase diagram of Fig. 1.…”

“…Such a weak shoulder has also observed previously in La 2Àx Sr x CuO 4 [12,14,27], where the holes in CuO 2 plane created by Sr doping have mainly O 2p and only little Cu 3d character. This is a result of the fact that to achieve a 3d 8 10 L final state in Cu L 2;3 XAS. A similar spectral feature has also been found in the Cu(III) oxide NaCuO 2 in which a 90°Cu-O-Cu interaction pathway causes the inter-plaquette interaction to be weak, leading to a behavior of analogous that an isolated CuO 4 plaquette [23].…”

“…For d ¼ 0 and x ¼ 2y, Cu and Ru have the valence states of Cu(II) and of Ru(IV), as in La 2 CuO 4 and Sr 2 RuO 4 , respectively. For x > 2y, thermogravimetric measurements have shown that up to 40% of the Ru or Cu cations adopt a higher oxidation state [7,8], which then is either due to formation of Cu(III) or Ru(V) ions. In the case of high values of the ratio x=2y, the system tends to become oxygen-deficient.…”

Structural Investigations of ACu3Ru4O12 (A=Na, Ca, Sr, La, Nd)—A Comparison between XRD-Rietveld and EXAFS Results

“…In a previous paper a comparison between Ru-K EXAFS and XRD measurements has been described for the system La 2Àx Sr x Cu 1Ày Ru y O 4Àd (5). Unfortunately, the compounds of this system possess a rather complex structure: Derived from the K 2 NiF 4 -structure type, data analysis is complicated by the fact that La/Sr and Cu/Ru share the A-and B-type cationic positions.…”

Determination of the Oxidation States of Cu and Ru in the System La2−xSrxCu1−yRuyO4−δ by XANES-Measurements

“…In principal, both Cu and Ru may become higher oxidized. In a previous article the examination of the local Cu and Ru environments by EXAFS (extended X-ray absorption "ne structure) spectroscopy was reported (6). One of the major results was that the oxygen vacancies are exclusively located in the copper surrounding while ruthenium remains in an octahedral coordination sphere.…”